1. Field of the Invention
The present invention relates to an extraction optical system, an optical pick-up device and an optical disc apparatus, and more particularly relates to an extraction optical system extracting a signal light component from a light in which the signal light component and a stray light component are mixed, an optical pick-up device including the extraction optical system and an optical disc apparatus provided with the optical pick-up device.
2. Discussion of the Background
Recently, accompanied by improvement of digital technology and advancement in data compression technology, an optical disc such as a DVD (digital versatile disc), etc. has come to attract attention as a medium for recording information, such as music, movie, photo, and computer software (hereinafter referred to as “contents” also), and concurrently with price-reduction of the optical disc, an optical disc apparatus using the optical disc as the medium for recording information has come into wide use.
The information volume of contents tends to increase year by year, and the recording capacity of the optical disc is expected to further increase. As a means to increase the recording capacity of the optical disc, a multi-layered recording layer is conceivable, and development of an optical disc having a plurality of recording layers (hereinafter referred to as “a multi-layered disc” also) and an optical disc apparatus geared to accessing the multi-layered disc is widely pursued.
In a multi-layered disc, if the interval between recording layers is broad, a signal from a selected recording layer deteriorates due to influence of spherical aberration, so that there is the tendency to narrow the interval between recording layers. However, if the interval between recording layers is narrowed, due to so-called cross talk between layers, not only a reflection at a target recording layer (hereinafter referred as “a signal light” also) but also a reflection at a recording layer other than the target recording layer (hereinafter referred to as “a stray light” also) are included at a high level in the return light from the multi-layered disc, so that the S/N ratio of a reproducing signal decreases.
With respect to the above-described problem, an apparatus in which the cross talk between layers is decreased when playing back a multi-layered disc has been proposed (see, for example, Japanese Patent No. 2624255).
In the apparatus proposed in JP No. 2624255, however, a signal detection optical system in which a pin hall is arranged to decrease the cross talk between layers and an astigmatism optical system detecting a disc position and a deviation in a condensing spot are individually necessary, so that it is relatively difficult to make the layout of the apparatus small. Thus, the apparatus has been unsuitable for the optical disc apparatus for which downsizing is desired. Further, an optical system described in the above-referred JP requires a light receiving element in each of the signal detection optical system and the astigmatism optical system, so that the cost is relatively high, which is inconvenient.